1. Field of the Invention
This invention relates to a printing machine which prints sheets from a normal size of B5 and the like to a small size of such as a post card and a visiting card, and more particularly to a printing machine provided with a flat platen.
2. Description of the Related Art
A conventional printing machine as shown in FIG. 1 comprises a printing head 2, a flat platen 3 facing the printing head 2, a pair of roller shafts 11 arranged at the front and rear sides of the flat platen 3 for passing the sheets in the space between the printing head 2 and the flat platen 3, pinch rollers 12 pressed against the corresponding roller shafts 11, a driving motor 13 for rotating the rear roller shaft 11 through a gear train 15, and sheet guides 14a and 14b and the like. A timing belt 16 is stretched between the roller shafts 11 so as to rotate the roller shafts 11 in the same direction at the same rotational speed.
Upon feeding various kinds of sheets in the direction X as shown in FIG. 1, this conventional printing machine is encountered with the following problems:
First, it is necessary to properly adjust the pressures exerted on the roller shafts 11 by the pinch rollers 12 in accordance with the thickness of the sheets. However, it is difficult to make an appropriate pressure adjustment which is suited for the sheets having different thicknesses. An insufficient pressure causes the roller shaft 11 to slip on the sheet 9, and an excessive pressure does not permit a thick sheet 9 to be inserted between the pinch roller 12 and the shaft roller 11, whereby the sheets 9 cannot be fed accurately in both cases.
Secondly, the provision of a damping rubber 4 on the printing face 3a of the flat platen 3 avoids the damage of the printing head 2 and lowers noise upon printing. Since, however, the printing heads 2 hits the same portion of the damping rubber 4 many times, the damping rubber 4 is worn out in a very short time.
The specification of the Japanese laid-open patent application No. 63-254078 discloses a printing machine in which a platen rubber is wound around driving rollers. It is convenient for this printing machine to feed a continuous sheet because the continuous sheet is fed by means of a sheet-feeding tractor, but this printing machine cannot accurately feed the sheets having different sizes ranging from the normal size of such as the B5 to the small size such as a post card.